Phosphorus is found in every cell and makes up 1% of a person's body weight. This element plays an essential role in sustaining life, such as the energy metabolism of cells. The concentration of phosphorus in blood is determined by absorption from the gastrointestinal tract and excretion from the kidney as well as bone formation and bone resorption and adjusted to a constant level. The phosphorus absorption in the gastrointestinal tract is performed mainly by a sodium-dependent phosphate transporter NaPi-IIb (SLC34A2) (Non-Patent Documents 1 and 2). Phosphorus in blood is filtered by renal glomerulus and reabsorbed in necessary amounts mainly by NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) in the renal tubule (Non-Patent Documents 1 and 3). The kidney plays a very important role in regulating phosphorus in vivo. In end-stage renal failure patients and dialysis patients with impaired renal functions, phosphorus accumulates in the body, resulting in a rise in phosphorus concentration in blood, i.e., hyperphosphatemia.
Hyperphosphatemia brings about the calcification of soft tissues. Particularly, vascular calcification is considered responsible for the dysfunction of the heart, leading to the death of the patient. Hyperphosphatemia also brings about the hypersecretion of parathyroid hormones, i.e., secondary hyperparathyroidism, and causes bone lesions. Thus, hyperphosphatemia is viewed as a problematic factor that deteriorates the prognosis and QOL of end-stage renal failure patients and dialysis patients.
In the current treatment of hyperphosphatemia, phosphorus adsorbents are used for the purpose of suppressing phosphorus absorption in the gastrointestinal tract. Nonmetallic polymer adsorbents typified by sevelamer carbonate and sevelamer hydrochloride, calcium salt preparations typified by precipitated calcium carbonate, or metallic adsorbents typified by lanthanum carbonate are used as the phosphorus adsorbents. These adsorbents, however, have each been reported to have adverse reactions such as gastrointestinal disorders including constipation and diarrhea, hypercalcemia caused by a rise in serum calcium concentration, and in vivo metal accumulation. In addition, these adsorbents require a daily intake of a few grams and therefore present noncompliance problems. Accordingly, there is a strong demand for the development of novel hyperphosphatemia therapy improved in terms of these problems of the phosphorus adsorbents.
The inhibition of NaPi-IIb, which plays a major role in phosphorus absorption in the gastrointestinal tract, may suppress phosphorus absorption in the gastrointestinal tract, as with the phosphorus adsorbents, to decrease phosphorus concentration in blood (Non-Patent Documents 2 and 4). Also, PiT-1 (SLC20A1) and PiT-2 (SLC20A2), which are sodium-dependent phosphate transporters like NaPi-IIb, are partially responsible for phosphorus absorption in the gastrointestinal tract (Non-Patent Documents 1 and 6). Thus, a compound that inhibits NaPi-IIb, PiT-1, and PiT-2 can be expected to produce a stronger phosphorus absorption inhibitory effect and decrease phosphorus concentration in blood, compared with an inhibitor for only NaPi-IIb. Meanwhile, the suppression of phosphorus absorption by the inhibition of these sodium-dependent phosphate transporters is based on the mechanism of action different from that of the phosphorus adsorbents currently used. Thus, the sodium-dependent phosphate transporter inhibitor can be expected to serve as a novel preventive or therapeutic agent for hyperphosphatemia in place of the conventional phosphorus adsorbents. The sodium-dependent phosphate transporter inhibitor is further expected to exert preventive or therapeutic effects on secondary hyperparathyroidism and chronic kidney disease by decreasing phosphorus concentration in blood (Non-Patent Document 5). Chronic kidney disease (CKD) is a disease which brings about persistent kidney damage (e.g., proteinuria) or persistent deterioration in kidney function (a decrease in GFR: glomerular filtration rate). In 2012, the number of patients having the disease in Japan reached 13, 300, 000 (Non-Patent Document 7), and it has been desired to develop a new medicament for preventing or treating chronic kidney disease.
NTX1942 (Patent Document 1) and compounds described in Patent Documents 2 and 4 have been reported so far as NaPi-IIb inhibitors. Also, a compound having a pyridazine skeleton has been reported in Patent Document 3 which makes mention about the treatment of anemia, ischemia, and hypoxia by the HIF hydroxylase inhibitory activity of the compound.